1 - Single click on the image to open it up in a new window. (If you use the image directly from the blog post you will lose a lot of resolution.)

2 - Right-click (or ctrl-click) on the image, and chose the option that says, "Set as Desktop Background" or "Use Image as Desktop Picture" or "Save Image As...". The wording may vary. (If saving the image to your computer is the only option, then locate it on your computer and choose the "Set as Desktop Background" or "Use Image as Desktop Picture" option from there.)

3 - If the image does not fit your desktop neatly, you may have to adjust the image (Mac: System Preferences - Desktop and Screen Saver - Desktop; Windows: Control Panel - Display - Desktop) and choose "Fill screen" as the display mode of your background image.